Wireless devices typically operate at one or more cellular communication standards, and/or wireless connectivity standards, and/or broadcast standards, each standard being allocated in one or more frequency bands, and said frequency bands being contained within one or more regions of the electromagnetic spectrum.
For that purpose, a typical wireless device must include a radiating system capable of operating in one or more frequency regions with an acceptable radio-electric performance (in terms of for instance input impedance level, impedance bandwidth, gain, efficiency, or radiation pattern). Moreover, the integration of the radiating system within the wireless device must be effective to ensure that the overall wireless device attains a good radio-electric performance (such as for example in terms of radiated power, received power, sensitivity or SAR (Specific Absorption Rate)) when human loading effects are considered.
Additionally, a space within the wireless device is usually limited and the radiating system has to be included in the available space. The radiating system is expected to be small enough to occupy as little space as possible within the device, which then allows for smaller devices, or for the addition of more specific components and functionalities into the device. At the same time, it is sometimes required for the radiating system to be flat since this allows for slim devices. Thus, many of the demands for wireless devices also translate to specific demands for the radiating systems thereof.
This is even more critical in the case in which the wireless device is a multifunctional wireless device. Commonly-owned patent applications WO2008/009391 and US2008/0018543, incorporated herein by reference in their entireties, describe a multifunctional wireless device.
For a good wireless connection, high efficiency is further required. Other more common design demands for radiating systems are the voltage standing wave ratio (VSWR) and the impedance which is supposed to be about 50 ohms.
Other demands for radiating systems for wireless handheld or portable devices are competitive cost and a low SAR.
Furthermore, a radiating system has to be integrated into a device or in other words, a wireless device has to be constructed such that an appropriate radiating system may be integrated therein which puts additional constraints by consideration of the mechanical fit, the electrical fit, and the assembly fit.
Of further importance, usually, is the robustness of the radiating system which means that the radiating system does not change its properties upon smaller shocks to the device and the human loading.
Besides electromagnetic functionality, small size, cost and reduced interaction with the human body (such as for instance SAR), one of the current limitations of the prior-art is that generally the antenna system is customized for every particular wireless handheld device model. The mechanical architecture of each model is different and the volume available for the antenna severely depends on the form factor of the wireless device model together with the arrangement of the multiple components embedded into the device (e.g., displays, keyboards, battery, connectors, cameras, flashes, speakers, chipsets, memory devices, etc.). As a result, the antenna within the device is mostly designed ad hoc for every model, resulting in a higher cost and a delayed time to market.
A radiating system for a wireless handheld or portable device typically includes a radiating structure comprising an antenna element which operates in combination with a ground plane layer providing a determined radioelectric performance in one or more frequency regions of the electromagnetic spectrum. This is illustrated in FIG. 1, in which it is shown a conventional radiating structure 10 comprising an antenna element 11 and a ground plane layer 12. Typically, the antenna element has a dimension close to an integer multiple of a quarter of the wavelength at a frequency of operation of the radiating structure, so that the antenna element is at resonance or substantially close to resonance at said frequency and a radiation mode is excited on said antenna element.
In some cases, the antenna element acting in cooperation with the ground plane does not attain sufficient impedance bandwidth as for covering multiple wireless standards and complex matching network must be added between the antenna element and the input/output port in order to increase said impedance bandwidth.
In addition, antenna elements operating in multiple frequency bands allocated at different regions of the electromagnetic spectrum usually presents a complex mechanical designs and considerable dimensions, mainly due to the fact that antenna performance is highly related to the electrical dimensions of the antenna element.
A further problem associated to the integration of the radiating structure, and in particular to the integration of the antenna element in a wireless device is that the volume dedicated for such an integration has continuously shrunk with the appearance of new smaller and/or thinner form factors for wireless devices, and with the increasing convergence of different functionalities in a same wireless device.
Some techniques to miniaturize and/or optimize the multiband behavior of an antenna element have been described in the prior art. However the radiating structures described therein still rely on exciting a radiation mode on the antenna element for each one of the frequency bands of operation. This fact leads to complex mechanical designs and large antennas that usually are very sensitive to external effects (such as for instance the presence of plastic or dielectric covers that surround the wireless device), to components of the wireless device (such as for instance, but not limited to, a speaker, a microphone, a connector, a display, a shield can, a vibrating module, a battery, or an electronic module or subsystem) placed either in the vicinity of, or even underneath, the radiating element, and/or to the human loading. A multiband antenna system is sensitive to any of the above mentioned aspects because they may alter the electromagnetic coupling between the different geometrical portions of the radiating element, which usually translates into detuning effects, degradation of the radio-electric performance of the antenna system and/or the radio-electric performance wireless device, and/or greater interaction with the user (such as an increased level of SAR).
In this sense, a radiating system such as the one described in the present invention not requiring a complex and/or large antenna formed by multiple arms, slots, apertures and/or openings and a complex mechanical design is preferable in order to minimize such undesired external effects and simplify the integration within the wireless device.
Some other attempts have focused on antenna elements not requiring a complex geometry while still providing some degree of miniaturization by using an antenna element that is not resonant in the one or more frequency ranges of operation of the wireless device.
For example, WO2007/128340, incorporated herein by reference in its entirety, discloses a wireless portable device comprising a non-resonant antenna element for receiving broadcast signals (such as, for instance, DVB-H, DMB, T-DMB or FM). The wireless portable device further comprises a ground plane layer that is used in combination with said antenna element. Although the antenna element has a first resonant frequency above the frequency range of operation of the wireless device, the antenna element is still the main responsible for the radiation process and for the electromagnetic performance of the wireless device. This is clear from the fact that no radiation mode can be excited on the ground plane layer because the ground plane layer is electrically short at the frequencies of operation (i.e., its dimensions are much smaller than the wavelength). For this kind of non-resonant antenna elements, a matching circuitry is added for matching the antenna to a level of VSWR in a limited frequency range which in this particular case can be around VSWR≤6. Such level of VSWR together with the limited bandwidth result in antenna elements which are only acceptable for reception of electromagnetic wave signals but not desirable for transmission of electromagnetic wave signals. With such limitations, while the performance of the wireless portable device may be sufficient for reception of electromagnetic wave signals (such as those of a broadcast service), the antenna element could not provide an adequate performance (for example, in terms of input return losses or gain) for a communication standard requiring also the transmission of electromagnetic wave signals.
Commonly-owned patent application WO2008/119699, incorporated herein by reference in its entirety, describes a wireless handheld or portable device comprising a radiating system capable of operating in two frequency regions. The radiating system comprises an antenna element having a resonant frequency outside said two frequency regions, and a ground plane layer. In this wireless device, while the ground plane layer contributes to enhance the electromagnetic performance of the radiating system in the two frequency regions of operation, it is still necessary to excite a radiation mode on the antenna element. In fact, the radiating system relies on the relationship between a resonant frequency of the antenna element and a resonant frequency of the ground plane layer in order for the radiating system to operate properly in said two frequency regions. Nevertheless, the solution still relies on an antenna element whose size is related to a resonant frequency that is outside of the two frequency regions but it is close to such frequency regions and on a complex matching network including resonators and filters for each frequency region of operation.
Other attempts for covering several frequency bands allocated in a particular frequency region of the electromagnetic spectrum rely on the use of antenna elements distributed along the ground plane of a wireless handheld or portable device as disclosed in a commonly-owned patent application WO2007/141187, incorporated herein by reference in its entirety. Each one of the antenna elements of said distributed antenna system resonates or substantially resonates at a frequency within a first frequency region of the electromagnetic spectrum. The antenna elements are combined by a phase shifting element that provides a phase difference among the radiating elements, which results in a wide bandwidth. According to the invention, the combination of two or more small antenna elements makes it possible to keep small the contribution of the ground-plane, which makes it possible to reduce the overall influence of the hand loading effects. Such combination of the antenna elements may not guaranty a balanced power distribution among the antenna elements and therefore the influence of the hand loading is dependent on its position on the said small antenna elements.
Another limitation of current wireless handheld or portable devices relates to the fact that the design and integration of an antenna element for a radiating structure in a wireless device is typically customized for each device. Different form factors or platforms, or a different distribution of the functional blocks of the device will force to redesign the antenna element and its integration inside the device almost from scratch.
For at least the above reasons, wireless device manufacturers regard the volume dedicated to the integration of the radiating structure, and in particular the antenna element, as being a toll to pay in order to provide wireless capabilities to the handheld or portable device.
In order to reduce as much as possible the volume occupied into the wireless handheld or portable device, recent trends in handset antenna design are oriented to maximize the contribution of the ground plane to the radiation process by using very small non-resonant elements. However, non-resonant elements usually are forced to include a complex radiofrequency system. Thus, the challenge of these techniques mainly relies on said complexity (combination of inductors, capacitors, and transmission lines), which is required to satisfy impedance bandwidth and efficiency specifications.
Commonly owned patent applications, WO2010/015365 and WO2010/015364, incorporated herein by reference in their entireties, are intended for solving some of the aforementioned drawbacks. Namely, they describe a wireless handheld or portable device comprising a radiating system including a radiating structure and a radiofrequency system. The radiating structure is formed by a ground plane layer presenting suitable dimensions as for supporting at least one efficient radiation mode and at least one radiation booster capable of coupling electromagnetic energy to said ground plane layer. The radiation booster is not resonant in any of the frequency regions of operation and consequently a radiofrequency system is used to properly match the radiating structure to the desired frequency bands of operation.
More particularly, in WO2010/015364 each radiation booster is intended for providing operation in a particular frequency region. Thus, the radiofrequency system is designed in such a way that the first internal port associated to the first radiation booster is highly isolated from the second internal port associated to a second radiation booster. Said radiofrequency system usually comprises a matching network including resonators for each one of the frequency regions of operation and a set of filters for each one of the frequency regions of operation. Thus, said radiofrequency system requires multiple stages and the performance of the radiating systems in terms of efficiency may be affected by the additional losses of the components. As each radiation booster is intended for providing operation in a particular frequency region, the bandwidth capabilities may be limited for some applications requiring very wide bandwidth specially at the low frequency region, as for example for wireless devices operating at LTE700, GSM850 and GSM900. Additionally, such radiating systems do not provide a redundancy mechanism for minimizing the human loading effects.
Another technique, as disclosed in U.S. Pat. No. 7,274,340, is based on the use of non-resonant elements where the impedance matching is provided through the addition of two matching circuits. Despite the use of non-resonant elements, the size of the element for the low band is significantly large, being 1/9.3 times the free-space wavelength of the lowest frequency for the low frequency band. Due to such size, the low band element would be a resonant element at the high band. Additionally, the operation of this solution is closely linked to the alignment of the maximum E-field intensity of the ground plane and the coupling element. The size of the low band element undesirably contributes to increase the printed circuit board (PCB) space required by the antenna module. According to the invention, the bandwidth at the low frequency region is 133 MHz (from 824 MHz to 954 MHz) that is insufficient for some applications requiring very wide bandwidth specially at the low frequency region, as for example for wireless devices operating at LTE700, GSM850 and GSM900. Additionally, such radiating systems do not provide a redundancy mechanism for minimizing the human loading effects
Therefore, a wireless device not requiring an antenna element and including a redundancy system, comprising several radiation boosters and a simple combining means would be advantageous to make simpler the integration of the radiating structure into the wireless device, increase the robustness to human loading effects and provide enhanced radio-electric operation to operate in more wireless services. The volume freed up by the absence of a large and complex antenna element would enable smaller and/or thinner devices, or even to adopt radically new form factors (such as for instance elastic, stretchable and/or foldable devices) which are not feasible today due to the presence of an antenna element featured by a considerable volume. Furthermore, by eliminating precisely the element that requires customization, a standard solution is obtained which only requires minor adjustments to be implemented in different wireless devices.